As the world's demand for electric energy increases, the existing power grid is being taxed beyond its ability to serve this demand continuously. In certain parts of the United States, inability to meet peak demand has led to inadvertent brownouts and blackouts due to system overload and deliberate “rolling blackouts” of non-essential customers to shunt the excess demand. For the most part, peak demand occurs during the daytime hours (and during certain seasons, such as summer) when business and industry employ large quantities of power for running equipment, heating, air conditioning, lighting, etc. During the nighttime hours, demand for electricity is often reduced significantly, and the existing power grid in most areas can usually handle this load without problem.
To address the lack of power at peak demand, users are asked to conserve where possible. Power companies often employ rapidly deployable gas turbines to supplement production to meet demand. However, these units burn expensive fuel sources, such as natural gas, and have high generation costs when compared with coal-fired systems, and other large-scale generators. Accordingly, supplemental sources have economic drawbacks and, in any case, can provide only a partial solution in a growing region and economy. The most obvious solution involves construction of new power plants, which is expensive and has environmental side effects. In addition, because most power plants operate most efficiently when generating a relatively continuous output, the difference between peak and off-peak demand often leads to wasteful practices during off-peak periods, such as over-lighting of outdoor areas, as power is sold at a lower rate off peak. Thus, it is desirable to address the fluctuation in power demand in a manner that does not require construction of new plants and can be implemented either at a power-generating facility to provide excess capacity during periods of peak demand, or on a smaller scale on-site at the facility of an electric customer (allowing that customer to provide additional power to itself during peak demand, when the grid is over-taxed).
Another scenario in which the ability to balance the delivery of generated power is highly desirable is in a self-contained generation system with an intermittent generation cycle. One example is a solar panel array located remotely from a power connection. The array may generate well for a few hours during the day, but is nonfunctional during the remaining hours of low light or darkness.
In each case, the balancing of power production or provision of further capacity rapidly and on-demand can be satisfied by a local back-up generator. However, such generators are often costly, use expensive fuels, such as natural gas or diesel fuel, and are environmentally damaging due to their inherent noise and emissions. Thus, a technique that allows storage of energy when not needed (such as during off-peak hours), and can rapidly deliver the power back to the user is highly desirable.
A variety of techniques is available to store excess power for later delivery. One renewable technique involves the use of driven flywheels that are spun up by a motor drawing excess power. When the power is needed, the flywheels' inertia is tapped by the motor or another coupled generator to deliver power back to the grid and/or customer. The flywheel units are expensive to manufacture and install, however, and require a degree of costly maintenance on a regular basis.
Another approach to power storage is the use of batteries. Many large-scale batteries use a lead electrode and acid electrolyte, however, and these components are environmentally hazardous. Batteries must often be arrayed to store substantial power, and the individual batteries may have a relatively short life (3-7 years is typical). Thus, to maintain a battery storage system, a large number of heavy, hazardous battery units must be replaced on a regular basis and these old batteries must be recycled or otherwise properly disposed of.
Energy can also be stored in ultracapacitors. A capacitor is charged by line current so that it stores charge, which can be discharged rapidly when needed. Appropriate power-conditioning circuits are used to convert the power into the appropriate phase and frequency of AC. However, a large array of such capacitors is needed to store substantial electric power. Ultracapacitors, while more environmentally friendly and longer lived than batteries, are substantially more expensive, and still require periodic replacement due to the breakdown of internal dielectrics, etc.
Another approach to storage of energy for later distribution involves the use of a large reservoir of compressed air. Storing energy in the form of compressed gas has a long history and components tend to be well tested, reliable, and have long lifetimes. The general principle of compressed-gas or compressed-air energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
By way of background, a so-called compressed-air energy storage (CAES) system is shown and described in the published thesis entitled “Investigation and Optimization of Hybrid Electricity Storage Systems Based Upon Air and Supercapacitors,” by Sylvain Lemofouet-Gatsi, Ecole Polytechnique Federale de Lausanne (20 Oct. 2006) (hereafter “Lemofouet-Gatsi”), Section 2.2.1, the disclosure of which is hereby incorporated herein by reference in its entirety. As stated by Lemofouet-Gatsi, “the principle of CAES derives from the splitting of the normal gas turbine cycle—where roughly 66% of the produced power is used to compress air-into two separated phases: The compression phase where lower-cost energy from off-peak base-load facilities is used to compress air into underground salt caverns and the generation phase where the pre-compressed air from the storage cavern is preheated through a heat recuperator, then mixed with oil or gas and burned to feed a multistage expander turbine to produce electricity during peak demand. This functional separation of the compression cycle from the combustion cycle allows a CAES plant to generate three times more energy with the same quantity of fuel compared to a simple cycle natural gas power plant.
Lemofouet-Gatsi continue, “CAES has the advantages that it doesn't involve huge, costly installations and can be used to store energy for a long time (more than one year). It also has a fast start-up time (9 to 12 minutes), which makes it suitable for grid operation, and the emissions of greenhouse gases are lower than that of a normal gas power plant, due to the reduced fuel consumption. The main drawback of CAES is probably the geological structure reliance, which substantially limits the usability of this storage method. In addition, CAES power plants are not emission-free, as the pre-compressed air is heated up with a fossil fuel burner before expansion. Moreover, CAES plants are limited with respect to their effectiveness because of the loss of the compression heat through the inter-coolers, which must be compensated during expansion by fuel burning. The fact that conventional CAES still rely on fossil fuel consumption makes it difficult to evaluate its energy round-trip efficiency and to compare it to conventional fuel-free storage technologies.”
A number of variations on the above-described compressed air energy storage approach have been proposed, some of which attempt to heat the expanded air with electricity, rather than fuel. Others employ heat exchange with thermal storage to extract and recover as much of the thermal energy as possible, therefore attempting to increase efficiencies. Still other approaches employ compressed gas-driven piston motors that act both as compressors and generator drives in opposing parts of the cycle. In general, the use of highly compressed gas as a working fluid for the motor poses a number of challenges due to the tendency for leakage around seals at higher pressures, as well as the thermal losses encountered in rapid expansion. While heat exchange solutions can deal with some of these problems, efficiencies are still compromised by the need to heat compressed gas prior to expansion from high pressure to atmospheric pressure.
It has been recognized that gas is a highly effective medium for storage of energy. Liquids are incompressible and flow efficiently across an impeller or other moving component to rotate a generator shaft. One energy storage technique that uses compressed gas to store energy, but which uses a liquid, for example, hydraulic fluid, rather than compressed gas to drive a generator, is a so-called closed-air hydraulic-pneumatic system. Such a system employs one or more high-pressure tanks (accumulators) having a charge of compressed gas, which is separated by a movable wall or flexible bladder membrane from a charge of hydraulic fluid. The hydraulic fluid is coupled to a bi-directional impeller (or other hydraulic motor/pump), which is itself coupled to a combined electric motor/generator. The other side of the impeller is connected to a low-pressure reservoir of hydraulic fluid. During a storage phase, the electric motor and impeller force hydraulic fluid from the low-pressure hydraulic fluid reservoir into the high-pressure tank(s), against the pressure of the compressed air. As the incompressible liquid fills the tank, it forces the air into a smaller space, thereby compressing it to an even higher pressure. During a generation phase, the fluid circuit is run in reverse and the impeller is driven by fluid escaping from the high-pressure tank(s) under the pressure of the compressed gas.
This closed-air approach has an advantage in that the gas is never expanded to or compressed from atmospheric pressure, as it is sealed within the tank. An example of a closed-air system is shown and described in U.S. Pat. No. 5,579,640, the disclosure of which is hereby incorporated herein by reference in its entirety. Closed-air systems tend to have low energy densities. That is, the amount of compression possible is limited by the size of the tank space. In addition, since the gas does not completely decompress when the fluid is removed, there is still additional energy in the system that cannot be tapped. To make a closed air system desirable for large-scale energy storage, many large accumulator tanks would be needed, increasing the overall cost to implement the system and requiring more land to do so.
Another approach to hybrid hydraulic-pneumatic energy storage is the open-air system. In this system, compressed air is stored in a large, separate high-pressure tank (or plurality of tanks). A pair of accumulators is provided, each having a fluid side separated from a gas side by a movable piston wall. The fluid sides of a pair (or more) of accumulators are coupled together through an impeller/generator/motor combination. The air side of each of the accumulators is coupled to the high pressure air tanks, and also to a valve-driven atmospheric vent. Under expansion of the air chamber side, fluid in one accumulator is driven through the impeller to generate power, and the spent fluid then flows into the second accumulator, whose air side is now vented to atmospheric, thereby allowing the fluid to collect in the second accumulator. During the storage phase, electrical energy can used to directly recharge the pressure tanks via a compressor, or the accumulators can be run in reverse to pressurize the pressure tanks. A version of this open-air concept is shown and described in U.S. Pat. No. 6,145,311 (the '311 patent), the disclosure of which is hereby incorporated herein by reference in its entirety. Disadvantages of open-air systems can include gas leakage, complexity, expense and, depending on the intended deployment, potential impracticality.
Additionally, it is desirable for solutions that address the fluctuations in power demand to also address environmental concerns and include using renewable energy sources. As demand for renewable energy increases, the intermittent nature of some renewable energy sources (e.g., wind and solar) places an increasing burden on the electric grid. The use of energy storage is a key factor in addressing the intermittent nature of the electricity produced by renewable sources, and more generally in shifting the energy produced to the time of peak demand.
As discussed, storing energy in the form of compressed air has a long history. However, most of the discussed methods for converting potential energy in the form of compressed air to electrical energy utilize turbines to expand the gas, which is an inherently adiabatic process. As gas expands, it cools off if there is no input of heat (adiabatic gas expansion), as is the case with gas expansion in a turbine. The advantage of adiabatic gas expansion is that it can occur quickly, thus resulting in the release of a substantial quantity of energy in a short time frame.
However, if the gas expansion occurs slowly relative to the time with which it takes for heat to flow into the gas, then the gas remains at a relatively constant temperature as it expands (isothermal gas expansion). Gas stored at ambient temperature, which is expanded isothermally, recovers approximately three times the energy of ambient temperature gas expanded adiabatically. Therefore, there is a significant energy advantage to expanding gas isothermally. Gas may be not only expanded but compressed either isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of more extreme temperatures and pressures within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during expansion and compression, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
In the case of certain compressed gas energy storage systems according to prior implementations, gas is expanded from a high-pressure, high-capacity source, such as a large underground cavern, and directed through a multi-stage gas turbine. Because significant expansion occurs at each stage of the operation, the gas cools down at each stage. To increase efficiency, the gas is mixed with fuel and ignited, pre-heating it to a higher temperature, thereby increasing power and final gas temperature. However, the need to burn fossil fuel (or apply another energy source, such as electric heating) to compensate for adiabatic expansion substantially defeats the purpose of an otherwise clean and emission-free energy-storage and recovery process.
While it is technically possible to provide a direct heat-exchange subsystem to a hydraulic/pneumatic cylinder, an external jacket, for example, is not particularly effective given the thick walls of the cylinder. An internalized heat exchange subsystem could conceivably be mounted directly within the cylinder's pneumatic side; however, size limitations would reduce such a heat exchanger's effectiveness and the task of sealing a cylinder with an added subsystem installed therein would be significant, and make the use of a conventional, commercially available component difficult or impossible.
Thus, the prior art does not disclose systems and methods for rapidly compressing and expanding gas isothermally in a manner that allows maximum use of conventional, low-cost components, and which operates in a commercially practicable yet environmentally friendly manner. Furthermore, energy storage and recovery systems could be more more widely deployed if they converted the work done by the linear piston motion directly into electrical energy or into rotary motion via mechanical means (or vice versa). In such ways, the overall efficiency and cost-effectiveness of the compressed air system would be increased.